The present invention relates to data communication systems and more particularly to spread spectrum data communication systems of the frequency hopping type.
Spread spectrum communication systems have been used in a variety of fields. In the communication system of this type, the transmitted bandwidth is much greater than the bandwidth or rate of the information to be transmitted. The carrier signal is modulated by some other function to widen or spread the bandwidth for transmission. The received signal is remapped into the original information bandwidth to reproduce a desired signal. The spread spectrum communication system has many useful advantages: a selective call is possible; since the power spectrum density is low, private communication is allowed; and it is little influenced by interference either due to multipath fading or jamming. From this standpoint, the spread spectrum system has found many uses, such as mobile communication systems, avionics systems, satellite communication systems, scatter communication systems of both the ionospheric and tropospheric type, direction finders and distance measuring equipment.
The spread spectrum systems can be categorized into a direct sequence system, a frequency hopping system, a time hopping system and a hybrid system which is a proper combination of the systems just mentioned. Of those communication systems, the frequency hopping system is frequently used in the field of mobile communication systems with a low traffic volume for a number of stations. Also frequency hopping systems can be employed in satellite communication systems and scatter type communication systems where a fading environment is present.
In the frequency hopping system a carrier frequency is shifted or jumped in discrete increments in a pattern dictated by a prepared code sequence, for instance, a PN (pseudo-noise) code, and M-sequence codes, Gold codes and the like, in synchronism with a change in the state of the codes. The resulting consecutive and time sequential frequency pattern is called a hopping pattern and the duration of each hopping frequency is called a chip. The transmitted information is embedded in the codes or embedded in each frequency of the carrier wave by a so-called FSK (frequency shift keying) modulation. The information signal thus spread-spectrum-modulated can be reproduced at a receiver.
In reproducing the information signal by the receiver, a synchronization acquistion process is first performed, in which the code pattern provided in the receiver is made accurately coincident with the code pattern generated in the transmitter in time position. Then, the spread spectrum signal is despread, and thereafter a well known demodulation is performed to extract the desired information. More particularly, a local reference signal of a frequency correspondingly determined by the same code pattern as that in the transmitter for every chip and received signal are mixed in a mixer in order to perform a correlation (despreading) process for converting the spread spectrum signal into the signal having a frequency bandwidth wide enough to extract the information. This system is described in detail in "Spread Spectrum Systems" by R. C. Dixon, published by John Wiley & Sons, Inc. in 1976. Following this despreading process, the desired information is extracted by usual demodulation techniques.
Such a system is not only useful in obtaining a proper coherent transmission in a fading environment, such as is present in mobile communications, satellite communications and scatter communications, the system is also jammer resistant.